The invention relates to a method and a device for correcting the thickness of a metal strip during rolling comprising a roll stand with adjusting elements to regulate the thickness of the strip and at least one take-up coiler.
During the rolling of metal strip, for example, strip made of aluminium or an aluminium alloy, in order to reduce their thickness after passing through a roll stand, the rolled metal strips are wounded onto a take-up coiler as a coil. One important quality feature of the rolled strip wound as a coil is among other things the thickness of the strip and its fluctuations.
The thickness of the metal strip has hitherto been measured using a radiometric measuring method and the adjusting element of the roll stand controlled depending on this value. In the radiometric measuring method, a detector arranged on one side of the metal strip is used to measure the radiation transmitted by the metal strip from an emitter arranged on the other side of the metal strip. The radiation measured by the detector is in this case dependent on the absorption in the metal strip which is especially determined by the thickness of the metal strip. The deviation of the radiometrically determined strip thickness from a desired value of the strip thickness is used as an input quantity for controlling adjusting elements of the roll stand to influence the thickness of the metal strip. However, the radiometrically determined measured thickness is dependent on further variables, for example, the alloy composition of the strip, the air density and air temperature in the measurement path, for example during blowing out or sucking in heated air, the fraction of coolant and lubricant mist in the measurement path, as well as the temperature of the rolled material and the system components to determine the thickness. Thus, it is absolutely essential to determine the variables and standardise the radiometric measuring method to the additional variables.
A disadvantage with the conventionally known method for correcting the strip thickness of a metal strip during rolling is that a determination of the variables and standardisation of the radiometric measuring method to these variables cannot take place comprehensively under rolling conditions. Thus, to correct the alloy-dependent absorption behaviour of the metal strip, it is necessary to measure the alloy composition on a casting sample by means of spark spectrometry and to calculate therefrom an absorption index for the alloy which is taken into account in the radiometric measuring method. Variations during sampling and measurement uncertainties during the spark spectrometry have the result that the radiometrically determined thickness is provided with a confidence interval which is to be taken into account according to the product requirement.
In order to standardise the radiometric measured values, on the one hand the electrical zero point of the detector signal with the emitter aperture closed is used and on the other hand, under full irradiation, that is without a sample in the measuring path, a balancing factor is determined which takes into account the absorption conditions in the measuring section at this time. This takes place automatically during changing the coil or every time the emitter aperture is open without measured material. The temperature variation in the measurement path with the rolled material is determined by measuring the temperature variation and an empirical weighting factor determined therefrom. It is not possible to record the integral temperature profile in the measurement path which exhibits substantial variations during rolling as a result of the hot rolled strip. Thus, the integral temperature profile cannot be taken into account during the radiometric thickness measurement.
Furthermore, in systems known from the prior art for verifying the radiometric measuring point, control standards are located in a hermetically encapsulated area in the immediate vicinity of the emitter, i.e., metal sheets whose absorption properties do not vary except as a result of temperature variations. By verifying the measured thickness values of these control standards, the absorption curve entered into the measuring system can be corrected. However, this adjustment can likewise only be made when the rolling process is interrupted.
As a result of the disadvantages of the hitherto known methods for correcting the thickness of a metal strip during rolling, which have been described above, the thickness tolerances currently required can only be met with difficulty.